In the oil extraction field, when well downhole pressure is sufficiently high, fluids contained therein naturally reach the surface. In this case, those wells are named flow well and oil production occurs by means of natural elevation. Thus, when well downhole pressure is relatively low, in order fluids contained therein reach surface or the production capacity is increased, it is required to employ any elevation artificial method.
Among those elevation artificial methods, the most widely used, particularly for onshore rigs, is the mechanical pumping system for oil extraction. This system is basically composed of three parts, which are interrelated, as described below.
The first part is called a mechanical pump unit, which is installed on the well surface and is basically comprised by a motor or engine, a gearbox and a piston rod-crank-rocker arm mechanism, which is responsible for transforming rotary movement into alternative movement, and transfer it to the system secondary part named rod column.
The rod column comprises a series of interconnected rods involved by a pipe column, also known as production column, both extending from surface to oil well downhole. The system third part is installed on the pipes column bottom edge, in case it is an alternative piston pump.
The alternative piston pump, also known as well downhole pump, comprises a case with its top edge connected to the tube column bottom edge and a case piston with its top edge connected to the rod column bottom edge. Besides, the alternative piston pump comprises a ball-seat type stationary valve, also known as foot valve, fixed to case bottom and a ball-seat type travelling valve, also known as exhaust valve, contained on piston bottom part.
The alternative ascending and descending movement produced at mechanical pumping unit is transferred to the rod column, and from there to alternative piston valve, making a contained fluid on the reservoir to be elevated by means of the pipe column to well's surface.
In more details, on a pumping cycle, during the ascending movement, a piston elevates a fluid column contained on case part above itself and generates a thrust over the stationary ball valve so making it elevating from its seat and float, in such a way the fluid is aspired from well to bottom chamber between the stationary valve and exhaust valve. Thus, the stationary valve serves as a fluid intake valve of an oil reservoir to an alternative piston pump. Yet, during its ascending movement, the exhaust valve is closed, i.e., with its ball under the seat, due to the fluid column pressure above itself.
During the ascending movement, the piston puts strength over fluid contained on bottom chamber, closing the stationary valve and opening the exhaust valve allowing bottom chamber fluid to travel to the piston inner chamber, which communicates with pipe column. Thus, the exhaust valve serves as an alternative piston pump fluid discharge valve for pipe column.
Over the years several improvements were developed for this mechanical pumping system. However, a major problem remains without a satisfactory solution, as described below.
It is known that the fluid contained inside the wells is made of a non homogeneous mixture of oil and gas. Gas parts are extremely harmful as they make an alternative piston pump inefficient, most of the times causing shutdowns on fluid elevation draining.
In more details, during the piston ascending movement and subsequent stationary valve opening, gas parts are admitted to bottom chamber. Those gas parts as they are less dense than oil accumulate on bottom chamber top part. Otherwise, it is not possible to open the exhaust valve during piston descending movement, as it happens in normal operation. This happens due to the fact that a gas is a compressible fluid, unlike oil. Thus, during piston descending movement, a gas does not impose necessary strength for the exhaust valve ball to float, and ends up being compressed. So the gas is compressed and expanded alternately without the exhaust valve opening, i.e., without bottom chamber gas discharge for piston inner chamber, thus interrupting the fluid elevation draining.
This problem arises a series of inconveniences. For example, for gas suction, a special operation should be performed, using an auxiliary pumping system. All this operation is time and cost demanding, besides other difficulties inherent to the process.
To avoid this problem the document BRPI0604983-4 proposes a gas-liquid separator installed on producing well downhole before the pump. Such separator comprises an external cylinder capsule divided in four distinct chambers, namely: intake chamber, separated fluid chamber, pumping chamber and gas chamber. The fluid containing oil and gas is collected for the intake chamber through a set of holes. Inside the instrument the fluid is forced downwards following along a spiral. Along this path, the gas, by gravitational and centrifugal effect tends to be separated from oil, being collected by holes located at a wall intersection of an intermediary concentric pipe with the spiral. Gas is then directed to the space between an intermediary concentric pipe inner wall and suction pipe external wall, where it is directed to a gas chamber and released inside the well by means of discharge holes. Descending oil flow, which had its gaseous fraction separated, reaches a separated fluid chamber. Inside that chamber the oil goes up through a suction pipe until reaching a pumping chamber, from where fluid is propelled by the original pumping system, for example, a mechanical pumping system with alternative piston pump.
As it can be seen, the proposed separator according to BRPI0604983-4 is an additional instrument installed on the original pumping system presenting several specific elements provided on a extremely complex setup. Therefore, this solution presents a high cost, besides requiring constant maintenance.
Aiming to provide a solution for shutdown on gas in fluid elevation draining, for oil extraction mechanical pumping systems, as well as aiming to present an alternative solution to that proposed in document BRPI0604983-4, the present invention proposes an improvement for alternative piston pump, characterized by its case containing at least one cavity provided along with its inner surface.